DRDUL  The idea sounds great in principle, but I couldn't get it to work. I launched the app, clicked the OK button at the prompt where it tells me it's running in demo mode, and then it froze. I had to force-quit. I tried it again with the same result, so I dragged it to the trash.  
(Version 1.7)

GORDON80  It run for 6 hours and found 1 missing song...

I came across this one, which seems to work fine:

Music Folder Files Not Added v1.1

It found hundreds of lost files within minutes, which I could add to my library or delete. Nice!

You find it here:

http://dougscripts.com/itunes/scripts/ss.php?sp=musicfolderfilesnotadded  
(Version 1.7b2)

BONI  seems to work great! should be a part of itunes itself.  
(Version 1.2)